The invention relates to methods of making electrodes for electrochemical systems, especially cathodes for lithium polymer batteries, and products prepared from the methods.
Modern electrochemical systems such as fuel cells, capacitors, sensors, and batteries can be constructed of electrochemical components that include a set of electrodes. In batteries, electrodes are typically made up of materials including an active material (i.e., an electrochemically-active material capable of undergoing reduction-oxidation), e.g., an oxide such as vanadium oxide, disposed in a polymeric matrix which may include an ionically-conductive polymer. The greater the amount of active material included in the electrode, the greater the capacity of the battery. Battery electrodes may further contain other useful materials including an electrically-conductive material (sometimes referred to as an xe2x80x9celectrically-conductive diluentxe2x80x9d) such as carbon, and an electrolyte salt such as lithium bis(trifluoromethanesulfonyl)imide, among various others.
Electrodes are often produced using standard coating techniques by dissolving or dispersing the active component, the electrically-conductive material, the electrolyte salt, and the polymer in a solvent and coating the solution onto a substrate. The materials are generally milled prior to being dispersed into the solvent and coated.
Some attempts have been made to prepare electrochemical components by extrusion methods. Such methods may include conditions of high shear and high temperature, making them prone to degrade materials of the electrochemical components, especially the polymer. See, e.g., U.S. Pat. Nos. 4,976,904 and 5,804,116 5,749,927. Some of these have used solvents, plasticizers, liquids, or softening agents to avoid these conditions.
There is an ongoing need for new and improved methods of preparing electrochemical components such as electrodes, e.g., cathodes. Particularly desirable methods would allow the production of electrochemical components without degrading the components, most preferably could be accomplished with minimal or no use of solvents, and would produce an electrochemical component having useful properties, including, preferably, a high loading of active material.
The invention relates to methods for producing components of electrochemical systems such as electrodes, using a reciprocating, single screw extruder (sometimes referred to herein as a xe2x80x9creciprocating extruderxe2x80x9d). Specifically, it has been found that electrode components can be processed, e.g., combined, melted, and/or extruded, using a reciprocating extruder to form electrodes useful in electrochemical systems such as batteries. According to the invention, a reciprocating single screw extruder is used to combine electrode components into an electrode, with excellent distributive mixing, with very useful properties, and preferably without the use of additional solvents. The temperature and shear conditions of the reciprocating single-screw extruder can be sufficiently vigorous to provide a useful and well-mixed, uniform mixture while still being sufficiently mild to avoid significant degradation of the electrode components, such as the polymer. Advantageously, as will be appreciated to the skilled artisan, the method can be used in a continuous process of producing electrodes, e.g., by mixing continuous streams of feed materials in a constant, continuous fashion to produce a continuous stream of extrudate.
Extrusion is a desirable alternative to conventional methods of preparing electrochemical components, such as methods involving solvent coating. Reciprocating extruders are known machinery for extruding plastic materials. A reciprocating extruder includes a screw component with interrupted flights and pins extending from the barrel that align with those interruptions and allow the screw to be reciprocated along its axis. The result is effective mixing with a large number of relatively low shear events, as opposed to typical non-reciprocating single or twin screw extruders, which rely on a relatively low number of high shear events.
In the past, extrusion methods for preparing electrochemical systems have been used with solvents, i.e., xe2x80x9csoftening solvents.xe2x80x9d Examples of solvents or xe2x80x9csoftening solventsxe2x80x9d that have been used are ethylene carbonate, propylene carbonate, dimethyl carbonate, dietheyl carbonate, dimethyl adipate, tetramethylene sulfone, gamma-butyrolactone, dimethylformamide, dioctyl phthlate, tetrahydrofuran, polyethylene glycol dimethyl ether, and polyethylene glycol.
Materials used to produce electrodes, especially cathodes, may preferably include an active agent, an electrically-conductive material, xe2x80x9cionically-conductivexe2x80x9d polymer, and an electrolyte salt. Other optional ingredients such as additional polymer or other additives may of course also be included. According to the invention, an electrode can be produced with solventless (xe2x80x9cneatxe2x80x9d) forms of materials, and without the addition of a separate solvent ingredient. That is, the invention contemplates processing the electrode components through a reciprocating extruder without any added solvent, and preferably with essentially no solvent or absolutely no solvent at all. As used herein, xe2x80x9csolventxe2x80x9d is given its generally accepted and understood meaning, and includes organic and inorganic liquids or plasticizers known to be used or useful to dissolve or soften other organic or inorganic materials, and specifically includes water. Reducing or eliminating solvent from a method of producing battery electrodes has obvious advantages, including environmental advantages in the production of less waste, and elimination of processing steps that are designed to remove the solvent after deposition of the electrode materials onto a substrate, with elimination of attendant costs, time, and labor. Furthermore, if the solvent is not essentially fully removed from the electrode it may also reduce mechanical integrity or stability of the electrode.
Electrodes can be produced according to the invention to exhibit excellent mixing of the active ingredient, electrically-conductive material, polymer, and salt, and can advantageously be prepared to have relatively high active material loadings of greater than about 50 weight percent, e.g., about 60 to 68, or up to 86 weight percent active material. Specifically, an embodiment of the invention involves a continuous, solventless process for compounding a well-mixed, highly filled polymeric material for an electrochemical electrode, e.g., a cathode. The process includes extruding the materials using a reciprocating, single-screw extruder such as the type sold under the trade name Buss Kneader(copyright). The feed arrangement of the different components into the extruder can be any arrangement that proves useful. In one embodiment of the invention, the components are fed to the extruder according to the following arrangement: an electrolyte salt is fed at the extruder""s feed throat; polymer is fed slightly downstream, and an electrically-conductive material and active material are fed further downstream. An extrudate of the processed components is removed from the far downstream end of the extruder, in a shape conducive to the end use application. The processing can be accomplished with reduced degradation of the polymer during processing as compared to other methods.
An aspect of the invention relates to a method for combining electrode components. The electrode components include an active material, an electrically-conductive material, an ionically-conductive polymer, an electrolyte salt, and no added solvent. The method includes processing the electrode components using a reciprocating single screw extruder. Preferably, the individual electrode components and their combined mixture will contain essentially no solvent.
Another aspect of the invention relates to a method for combining electrode components that include an active material, an electrically-conductive material, an ionically-conductive polymer, and an electrolyte salt. The method includes processing an electrode components using a reciprocating single screw extruder and processing ionically-conductive polymer salt complex in a molten state. This is in contrast, for example, to methods of using an extruder to process a solvent-containing mixture for coating.
Yet a further aspect of the invention relates to a method of producing a battery cathode. The method includes processing a mixture of ingredients that include greater than about 50 weight percent active material; from about 1 to about 10 weight percent electrically-conductive material comprising carbon black, graphite, or a combination thereof; from about 10 to about 40 weight percent polymer comprising ionically-conductive, e.g., polyalkylenoxide, polymer; and from about 3 to about 15 weight percent fluorinated lithium salt. Also according to this aspect of the invention, the mixture can include a total of less than about 0.5 percent by weight solvent. The method involves a reciprocating single-screw extruder, and an ionically-conductive polymer salt complex is processed in a molten state.